When a slider assembly including a circulation maintaining device and a slider is mounted astride a slide rail, a plurality of closed circulation channels are formed jointly by the circulation maintaining device, the slider, and the slide rail. The circulation channels include a load channel formed between the slider and the slide rail, two turning channels respectively formed at two end surfaces of the slider, and return channels in the slider. Rows of rolling elements are received in the circulation channels and can roll in the circulation channels in a circulating manner without limitation in distance, thereby enabling the slider assembly to move smoothly along the slide rail.
More specifically, the load channel in the circulation channels is formed jointly by retaining members provided by the circulation maintaining device, rail surfaces of the slider, and rail surfaces of the slide rail. The retaining members have generally the same length as the rail surfaces of the slider, are shaped as long, narrow pieces, and generally include upper retaining members, middle retaining members, and lower retaining members. The upper and lower retaining members, which are located above and below the rolling elements respectively, can be fixed or connected in their middle sections, from above and below respectively, for enhanced rigidity. The middle retaining members, however, which are located at half-height of the slide rail and therefore subject to limitation of space between the slide rail and the slider, are connected and secured at the end portions, as is typical of the conventional circulation maintaining devices (see U.S. Pat. No. 6,729,760 “Motion guide device” and US Patent Application Publication No. 20030035600A1 “Rolling element interference preventer and a guide device” for example). As the rolling elements tend to push one another and push against the retaining members while rolling in the load channel, it is likely that the rolling elements will fall out somewhere between the two ends of each middle retaining member, causing damage to the slider assembly.
To increase the rigidity of a middle retaining member in its middle section, U.S. Pat. No. 7,204,036B2 “Linear guide apparatus” and U.S. Pat. No. 8,123,408B2 “Linear motion guide unit” disclose supporting, and thus enhancing the rigidity of, a middle retaining member by means of an elongated component made of a high-rigidity material such as steel. However, as the high-rigidity component can be fixed only at its two ends too, the improvement is limited, and this solution does not respond well to the trend of having increasingly longer slider rigid bodies. To overcome this shortcoming, U.S. Pat. No. 5,129,736 “Indefinite linear motion guide unit having four endless circulating paths” proposes passing a bolt transversely through each of the two solid feet of a slider from outside so as to fasten each middle retaining member to the slider at a position between two rail surfaces. However, not only is it difficult to make threaded through holes in the two feet of a solid rigid body, but also the number of components and the complexity of assembly will be increased. All of these contribute to a rise in production cost.